When a data pulse is applied to a laser diode, it switches the diode between a condition of OFF where very little current is flowing through the diode and there is no substantial lightwave output signal and an ON condition where a large amount of current is flowing through the diode causing it to lase and produce a lightwave or optical output signal. As the laser is changing between its OFF condition to its ON conditions, there is a shift in wavelength which shift is called "chirp". This term is borrowed from radar technology. This wavelength shift ceases very soon in the time it takes to change from an OFF to an ON condition but can produce enough components of frequencies other than the basic laser frequency to render detection of logic 0 and logic 1 states at a remote site difficult or impossible.
It is therefore essential that this chirp be held to manageable limits while still maximizing the laser output signal within the power limits of the diode.
A previous attempt to provide this control was illustrated in a Fraser U.S. Pat. No. 4,789,987, assigned to the same assignee as the present invention. This patent used amplitude modulation of two tones superimposed upon the data signal. However, the threshold position using this approach is sensitive to differential gain variation and an approach needed to be found which would provide a more stable control environment over a wider range of situations.
It is known in the art that the injection of two signals, such as is provided in the referenced Fraser patent, produces intermodulation products which would appear as sidebands on either side of the higher frequency signal of the two superimposed signals. All known prior art approaches to control have at a minimum ignored these products or in more sophisticated approaches, have tried to minimize or eliminate the products. The present invention, however, takes advantage of the intermodulation products to generate a signal proportional to the amplitude of the intermodulation product sidebands to adjust the amplitude of the data signal. Separate control loops are used to maintain the average power output and to normalize the amplitude of the 500 hertz signal used in the intermodulation process. The control of the amplitude of the data signal operates to control the level of the logic 0 portion of the data signal in accordance with a reference applied to that loop. The approach described minimizes the effects of the laser threshold position due to short term and long term gain variation in the laser and in the external circuitry.
It is an object of the present invention to provide an improved data modulation amplitude control of a laser circuit.